There has to have been an enemy around the dog tag to make it a 'denied kill' if I am not mistaken. So you must have been missing the enemy runs by and collect them ...  I don't play KC much so I wouldn't know.

I Play kc a lot so I kno wats happening with u the person u kills leaves the game and gives u a kill denied headsup besides that I have also have had the tag disappear into thin air

I only play HCKC, well mostly, and this is either:

1. Someone dashboards faster then you can get there, thus "kill denied"

2. Someone else kills that person just as they spawn, causing them to leave another tag in said location, causing the tag you had your eyes on to disappear and thus "kill denied"

Yes there are other inconsistencies in this game, but in KC those are causes for which the tags disappear the most.

Alot of times, if the person you killed has a tac insert placed, someone on your team could be camping it, and as soon as you kill them, your team member kills them and they get the tag you was running for. A player cannot have two sets of tags out at the same time.